1. Field of the Invention
The present invention generally relates to a copper alloy plate, such as a copper alloy sheet, and a method for producing the same. More specifically, the invention relates to a plate material, such as a sheet material, of a copper alloy containing titanium (a sheet material of a Cu—Ti alloy), which is used as the material of electric and electronic parts, such as connectors, lead frames, relays and switches, and a method for producing the same.
2. Description of the Prior Art
The materials used for electric and electronic parts, such as connectors, lead frames, relays and switches, are required to have such a high strength that the materials can withstand stress applied thereto during the assembly and operation of electric and electronic apparatuses using the parts. The materials used for electric and electronic parts, such as connectors, are also required to have an excellent bending workability since the parts are generally formed by bending. Moreover, in order to ensure the contact reliability between electric and electronic parts, such as connectors, the materials used for the parts are required to have an excellent stress relaxation resistance, i.e., a resistance to such a phenomenon (stress relaxation) that the contact pressure between the parts is deteriorated with age.
In recent years, there is a tendency for electric and electronic parts, such as connectors, to be integrated, miniaturized and lightened. In accordance therewith, the sheet materials of copper and copper alloys serving as the materials of the parts are required to be thinned, so that the required strength level of the materials is more severe. Specifically, the 0.2% yield strength of the materials is desired to be the strength level of not less than 850 MPa, preferably not less than 900 MPa, and more preferably not less than 950 MPa.
In accordance with the miniaturization and complicated shape of electric and electronic parts, such as connectors, it is required to improve the precision of shape and dimension of products manufactured by bending the sheet materials of copper alloys. As the requirements for products manufactured by bending the sheet materials of copper alloys, it is not only important to produce no cracks in the bent portions of the products, but it is also important to ensure the precision of shape and dimension of the products. Moreover, there is a problem in that spring back occurs in the bending of the sheet materials of copper alloys. Furthermore, the “spring back” means such a phenomenon that the shape of a final product is not in conformity with the shape of a product worked in a die since the resilient recovery of the product occurs when the product is taken off from the die after the working of a sheet material.
In particular, the problem on the spring back is easy to be explicit as the required strength level of the materials is more severe. For example, there are some cases where the shape and dimension of connector terminals having portions manufactured by box-bending are deviated, so that the connector terminals can not be used. For that reason, there is recently often applied a so-called bending after notching wherein a sheet material is bent along a notch which is formed by carrying out notching (working for forming the notch) in a portion of the sheet material. However, in the bending after notching, portions near the notch portion are work-hardened by notching, so that cracks are easily produced in the subsequent bending operation. Therefore, the bending after notching is a very severe bending process for materials.
Moreover, the requirements for the stress relaxation resistance of electric and electronic parts, such as connectors, are more severe as the increase of cases where the parts are used in severe environments. For example, the stress relaxation resistance of electric and electronic parts, such as connectors, is particularly important when the parts are used for automobiles in high-temperature environments. Furthermore, the stress relaxation resistance is such a kind of creep phenomenon that the contact pressure on a spring portion of a material forming electric and electronic parts, such as connectors, is deteriorated with age in a relatively high-temperature (e.g., 100 to 200° C.) environment even if it is maintained to be a constant contact pressure at ordinary temperature. That is, the stress relaxation resistance is such a phenomenon that the stress applied to a metal material is relaxed by plastic deformation produced by the movement of dislocation, which is caused by the self-diffusion of atoms forming a matrix and the diffusion of the solid solution of atoms, in a state that the stress is applied to the metal material.
However, there are trade-off relationships between the strength and bending workability of a sheet material of a metal and between the bending workability and stress relaxation resistance thereof, respectively, and conventionally, a sheet material having a good strength, bending workability or stress relaxation resistance is suitably chosen in accordance with the use thereof as a material used for a current-carrying part, such as a connector.
Among the sheet materials of copper alloys, the sheet materials of Cu—Ti alloys have the next highest strength to the sheet materials of Cu—Be alloys and a superior stress relaxation resistance to that of the sheet materials of Cu—Be alloys, and are more advantageous than the sheet materials of Cu—Be alloys in view of costs and environmental loads. For that reason, the sheet materials of Cu—Ti alloys (e.g., C199 (Cu-3.2 wt % of Ti)) are substituted for part of the sheet materials of Cu—Be alloys to be used as connector materials and so forth. However, it is known that the sheet materials of Cu—Ti alloys have a lower strength than that of the sheet materials of Cu—Be alloys (e.g., C17200) having a high strength if they have the same bending workability as that of the sheet materials of Cu—Be alloys, and that the sheet materials of Cu—Ti alloys have an inferior bending workability to that of the sheet materials of Cu—Be alloys if they have the same strength as that of the sheet materials of Cu—Be alloys.
As methods for improving the strength of the sheet materials of Cu—Ti alloys, there are a method for increasing the content of Ti, and a method for choosing a high temper material. However, in the method for increasing the content of Ti, if the concentration of Ti in a sheet material of a Cu—Ti alloy is too high (for example, if the content of T1 is not less than 5 wt %), cracks are easily produced in the sheet material during hot rolling and cold rolling, so that the productivity of the sheet material is remarkably deteriorated. In addition, large deposits are easily produced, so that the sheet material being the final product can not be utilized as a material for general electric and electronic parts since the bending workability of the sheet material is deteriorated although the strength thereof is high. On the other hand, in the method for choosing a high temper material, the strength of the sheet material is improved by increasing the rolling reduction before and after an ageing treatment, so that the sheet material being the final product has anisotropy although the strength thereof is high. That is, it is known that the bending workability of the sheet material in a direction perpendicular to a rolling direction (i.e., the bending workability of the sheet material in a so-called “bad way” in which the bending axis of the sheet material is parallel to the rolling direction) is remarkably bad, although the bending workability of the sheet material in a direction parallel to the rolling direction (i.e., the bending workability of the sheet material in a so-called “good way” in which the bending axis of the sheet material is perpendicular to the rolling direction on the rolled surface) is relatively good.
Generally, in order to improve the bending workability of the sheet materials of copper alloys, a method for fining the crystal grains of the copper alloys is effective. This is the same in the case of the sheet materials of Cu—Ti alloys. However, since the area of grain boundaries existing per a unit volume is increased as the crystal grain size is decreased, it is caused to promote stress relaxation being a kind of creep phenomenon if the crystal grains are fined. In addition, in sheet materials used in relatively high-temperature environments, the diffusion rate along the grain boundaries of atoms is far higher than that in the grains, so that there is a problem in that the original stress relaxation resistance of the sheet materials is deteriorated if the crystal grains are extremely fined (e.g., if the crystal grains are fined so as to have a grain size of 5 μm or less).
In particular, the sheet materials of Cu—Ti alloys have characteristics wherein deposits exist mainly in the form of a modulation structure (spinodal structure) in crystal grains, and the amount of deposits of particles in the second phase having the function of pinning the growth of recrystallized grains is relatively small, so that a mixed grain structure is easily caused by a difference in generating time of recrystallized grains during a solution treatment. Therefore, it is not easy to generate uniform and fine crystal grains.
In recent years, as methods for improving the characteristics of the sheet materials of Cu—Ti alloys, there are proposed a method for fining crystal grains and a method for controlling crystal orientation (texture) (see, e.g., Japanese Patent Laid-Open Nos. 2002-356726, 2004-231985, 2006-241573 and 2006-274289).
In Cu—Ti alloys, Ti exists in two forms, one of which is the form of a modulation structure (spinodal structure) having a periodical variation in concentration in a parent phase, and the other of which is the form of an intermetallic compound of Ti and Cu which are particles in the second phase (beta phases). The modulation structure is a structure which is generated by continuous fluctuation in concentration of Ti solute atoms and which is generated while holding the complete consistency with the parent phase. The sheet materials of Cu—Ti alloys having such a modulation structure are remarkably hardened, and have a small loss of ductility (bending workability). On the other hand, beta phases are deposits which are sprinkled in usual crystal grains and grain boundaries. Such beta phases are easily coarsened, and cause a remarkably great loss of ductility of the sheet materials although the function of hardening the sheet materials is extremely small by the modulation structure.
That is, in order to obtain the sheet materials of Cu—Ti alloys having both of a high strength and a good bending workability, it is effective to develop the modulation structure of the sheet materials while suppressing the generation of beta phases thereof. In addition, another important factor influencing on the bending workability of the sheet materials of Cu—Ti alloys is the crystal grain size of the alloys. As the crystal grain size of the alloys is decreased, the strain due to bending deformation can be dispersed to improve the bending workability of the sheet materials.
However, the crystal grain sizes of the sheet materials of Cu—Ti alloys are determined by recrystallization in the final solution treatment, and there is a problem in that the crystal grains are easily coarsened if the generation of beta phases having the function of pinning the growth of recrystallization is suppressed. In addition, the sheet materials of Cu—Ti alloys have such characteristics that the mixed grain structure is easily caused by a difference in generating time of recrystallized grains during a solution treatment. Therefore, it is not easy to generate uniform and fine crystal grains, so that cracks are easily produced near the boundaries of structures having different crystal grain sizes during bending deformation. Moreover, there is a problem in that the anisotropy in bending workability is easily caused if the rolling reduction before and after an ageing treatment is increased in order to improve the strength of the sheet materials of Cu—Ti alloys.
As a typical method for fining the crystal grains of the sheet material of a copper alloy having a chemical composition, there is a method for carrying out a solution treatment at a temperature of not higher than the solid solubility curve (solvus) of the copper alloy having the chemical composition. If the crystal grains of the sheet materials of Cu—Ti alloys are fined by this method, the solid solution of the total amount of Ti is not formed, and part of Ti is caused to remain as beta phases having the function of pinning. Therefore, although the crystal grains can be fined, the effect of improvement of bending workability due to the fining of crystal grains is offset by residual beta phases.
For example, in the method disclosed in Japanese Patent Laid-Open No. 2002-356726, a solution treatment is carried out at a lower temperature than the solid solubility curve of an alloy having a chemical composition by 10 to 60° C., so that the sheet material of a Cu—Ti alloy having a 0.2% yield strength of about 900 MPa can be obtained, but the ratio R/t of the minimum bending radius R to the thickness t of the sheet material in bending in the bad way remains a relatively great value of about 5.
In the method disclosed in Japanese Patent Laid-Open No. 2004-231985, Fe, Co, Ni and so forth are added to Cu—Ti alloys to form the intermetallic compounds of additional elements, such as Ti and Fe, so that the intermetallic compounds pin the boundaries of recrystallized grains to fine crystal grains in place of beta phases. However, there are disadvantages in that the development of the modulation structure of Ti is inhibited by the formation of the intermetallic compounds of the third element, such as Fe, and Ti, so that it is not possible to sufficiently improve the characteristics.
In the method disclosed in Japanese Patent laid-Open No. 2006-241573, the ratio of the intensity of X-ray diffraction on the {220} plane of a sheet material to that on the {111} plane thereof is set to be I{220}/I{111}>4 in order to improve the strength and electric conductivity of sheet material. If the rolling texture of the sheet material is adjusted so that the sheet material has a principal orientation component of {220}, it is effective to improve the strength and electric conductivity of the sheet material. However, it was found that the {220} plane was the rolling texture, so that the bending workability of the sheet material in the bad way was remarkably deteriorated.
In the method disclosed in Japanese Patent Laid-Open No. 2006-274289, in order to improve the bending workability of sheet materials, the texture of the sheet materials is controlled so that the maximum value of the intensities of X-ray diffraction in four regions including {110}<115>, {110}<114> and {110}<113> on a {111} positive pole figure is in the range of from 5.0 to 15.0 (a ratio of intensity to a random orientation). In addition, in order to obtain such a texture, the cold-rolling reduction is set to be in the range of from 85% to 97% before a solution treatment. Such a texture is a typical alloy-type texture ({110}<112>−{110}<100>), and the {111} positive pole figure thereof is similar to the {111} positive pole figure of 70/30 brass (see, e.g., “Metal Data Book”, third revision, p 361, edited by Japan Society for Metals, published by Maruzen Corporation). Thus, in the conventional method for adjusting the distribution in crystal orientation on the basis of the typical texture, it is difficult to greatly improve the bending workability of the sheet materials. In the above-described method disclosed in Japanese Patent Laid-Open No. 2006-274289, it is possible to obtain a sheet material of a Cu—Ti alloy having a 0.2% yield strength of about 870 MPa, but the ratio R/t of the minimum bending radius R to the thickness t of the sheet material in bending remains a relatively great value of about 1.6.
In order to improve the precision of shape and dimension of products manufactured by bending, it is effective to use the bending after notching for the sheet materials of copper alloys. However, in the sheet materials of Cu—Ti alloys wherein crystal grains and textures are controlled by the above-described methods disclosed in Japanese Patent Laid-Open Nos. 2002-356726, 2004-231985, 2006-241573 and 2006-274289, it was not considered to prevent cracks from being produced by the bending after notching, so that it was found that the bending workability after notching was not sufficiently improved.
In the sheet materials of Cu—Ti alloys, there is also a problem in that it is difficult to ensure the precision of shape and dimension of products, which are manufactured by bending, due to spring back. The bending after notching is effective in order to reduce spring back. In the bending after notching, portions near the notched portion are work-hardened by notching, so that cracks are easily produced in the subsequent bending. For that reason, in the sheet materials of Cu—Ti alloys, the bending after notching has not been adopted industrially in the present circumstances.
Moreover, if crystal grains are fined as described above, it is disadvantageous in order to overcome a stress relaxation being a kind of creep phenomenon although it is effective in order to improve the bending workability of the sheet materials to some extent. Thus, it is difficult to improve the stress relaxation resistance of the sheet materials in circumstances where it is difficult to sufficiently improve the bending workability thereof.